Problem: 9 pencils cost $13.41. Which equation would help determine the cost of 12 pencils?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 9 pencils. We want to know the cost of 12 pencils. We can write the numbers of pencils as a proportion: $\dfrac{9}{12}$ We know 9 pencils costs $13.41. We can let $x$ represent the unknown cost of 12 pencils. The proportion of these costs can be expressed as: $\dfrac{\$13.41}{x}$ The cost changes along with the number of pencils purchased, and so the two proportions are equivalent. $\dfrac{9}{12} = \dfrac{\$13.41}{x}$